Inertia is the property of an object that describes:

Inertia is the property of an object that basically describes its resistance to change its state of motion.

For instance, if an object is still, inertia describes the "attitude" of the object to stay still (a force should be applied in order to move it). Similarly, if an object is moving by uniform motion (with constant speed), inertia refers to the "attitude" of the object to keep its uniform motion (again, a force should be applied to the object in order to change this state of motion).